It has conventionally been practiced that a touch panel is disposed at frontage of LCD, organic EL, CRT, or other displays, which are widely used in such products as PDAs (Personal Digital Assistants), portable telephones, and personal computers. Basically, the touch panel is so structured that an upper electrode plate in which an upper electrode made of a transparent conductive film is formed on its lower surface, and a lower electrode plate in which a lower electrode made of a transparent conductive film is provided on its upper surface, are adhesively fixed only at their non-display area portions with the two electrodes opposed to each other via an air layer, wherein by pressing a part of a panel surface, the two electrodes insulated from each other by the air layer can be put into contact with each other and thereby into conduction, thus allowing an input operation to be performed. Also, each of the upper electrode plate and the lower electrode plate is formed of a single layer or by laminating a plurality of layers all over.
Then, for outdoor use, in order to prevent reflection of external light to improve visibility, a low-reflection touch panel can be used in which upper electrode plate 1 is formed by laminating, in an order from an upper electrode 2 side, at least a quarter wave plate 9 and a polarizing plate 10 whose absorption axis crosses an optical axis of the quarter wave plate 9 at an angle of 45° or 135° (see FIG. 4). That is, an anti-reflection filter of circular polarization type is formed by the quarter wave plate 9 and the polarizing plate 10 so that reflection of light incident from an external source at a transparent conductive film is efficiently cut. In addition, in a case where the display is an LCD, with only one quarter wave plate 9, even linearly polarized light coming incident for display use from an LCD side would be changed into circularly polarized light, and for this reason, a quarter wave plate whose optical axis perpendicularly crosses the quarter wave plate 9 in the upper electrode plate is further disposed in a lower electrode plate 3 to cancel a phase. That is, after being changed into circularly polarized light by the quarter wave plate on a lower electrode 4 side, the incident light is turned back to original linearly polarized light by the quarter wave plate 9 on the upper electrode 2 side.
Recently, in the automobile industry, there have been wide-spreading car navigation systems, which are used mainly by screen operation by remote control, and there have been discussed use of a touch panel on a display screen for more comfort of operability. In this case, there is an essential need for a touch panel having the aforementioned anti-reflection filter of a circular polarization type to prevent deterioration of visibility due to reflection of outdoor light such as sunlight. However, under summer's direct sunlight, vehicle interiors with windows closed may undergo a high-temperature environment over 70° C. In such a case, if left as it is for a long time, the touch panel would expand, whereby, since the quarter wave plate 9 and the polarizing plate 10 in the upper electrode plate 1 differ from each other in a stretching direction in a process of their axis formation, there is a fear that those plates result in mutually different expansional directions 11 during a state of their being bonded together directly or indirectly all over (see FIG. 5), thereby causing occurrence of waviness or strain in the upper electrode plate 1 of the touch panel. Also, under a high-temperature environment, since the quarter wave plate 9 in the upper electrode plate 1 is free to expand at central portions while blocked or, conversely, forcedly expanded by an effect of partial bonding and fixation with the lower electrode plate 3 in peripheral portions, there is a fear that stress 12 may be applied only to peripheral portions of the quarter wave plate 9 (see FIG. 6), causing the quarter wave plate 9 to be changed in retardation value in vicinities of peripheral portions of a display area and, as a result, impairing anti-reflection characteristics.
Therefore, for use in car navigations, there has been provided a touch panel in which a glass plate 13 is disposed in upper electrode plate 1 and bonded with quarter wave plate 9 all over with a view to avoiding the above-described problems (see FIG. 7). By bonding the glass plate with the quarter wave plate 9 all over in the upper electrode plate 1, the quarter wave plate 9 and polarizing plate 10 become almost free from occurrence of expansion even under a high-temperature environment over 70° C., thus held in a non-wavy, non-strained state. Also, by bonding this high-rigidity glass plate with the quarter wave plate 9 all over, stress applied to the quarter wave plate 9 is dispersed so that the quarter wave plate 9 becomes almost free from occurrence of retardation changes, thus losing anti-reflection characteristics less.
However, the upper electrode plate 1 is a part that makes direct contact with a finger or pen or the like, and in a case where a glass plate is used in the upper electrode plate 1, increased thickness of the upper electrode plate 1 would cause an input operation with the pen or finger to feel heavier because of high rigidity of the glass plate. For comfortable input operation to be performed, it is necessary to set a glass thickness to an extremely thin one. Unfortunately, with use of a glass plate having such a thickness, the touch panel would become highly liable to fracture, so that the touch panel would become quite difficult to handle during its assembly, and moreover, this glass could fracture during an input operation of the touch panel.
Accordingly, with a view to solving these issues, an object of the present invention is to provide a high-durability touch panel which is free from occurrence of deformation of its appearance configuration as well as deterioration of its anti-reflection characteristics due to retardation changes even if left under a high-temperature environment for a long time, and which allows comfortable input operations to be performed, and yet which is successful in workability during an assembly process.